1. Field of the Invention
This invention relates to a phase-change optical disc, and more particularly to a phase-change optical disc that is adapted to weaken a self-sharpening effect.
2. Description of Prior Art
Recently, recording media, such as a compact disc(CD) of write-once type, a recordable compact disc(CD-R), a rewritable compact disc(CD-RW), a magneto-optical disc(MOD), a pulse-change dual disc(PD), a digital versatile disc-random access memory(DVD-RAM) and so on, are commercially available. The discs such as Power Disc, CD-R and DVD-RAM have a phase-change characteristic so that an information can be repeatedly recorded. In such a phase-change optical disc, its combined structure is changed into an amorphous or crystal state by a laser light beam having a relatively great energy, thereby recording or erasing an information.
As shown in FIG. 1, the phase-change optical disc includes a first dielectric layer 4 made from ZnS--SiO.sub.2, a recording layer 6 made from GeSb--Sb.sub.2 Te.sub.3, a second layer 8 made from ZnS--SiO.sub.2, a reflective layer 10 made from a metallic material such as Al or Au, and a protective layer 12 made from a ultraviolet hardening resin, which are disposed on the upper portion of a poly-carbonate substrate 2, in turn.
In the phase-change disc having a four-layer film structure in this manner, the recording layer 6 is reversibly changed between the crystal state and the amorphous state by a laser light beam to record or erase an information. Herein, the amorphous state means recording pits having a logical value of `1` while the crystal state means recording pits having a logical value of `0` or an unrecorded state. The amorphous state is formed by heating the recording layer at a temperature higher than a melting point and thereafter cooling it. A change from the amorphous state into the crystal state, that is, an erasure is performed by heating the recording layer at a temperature higher than the crystallization temperature of the recording layer. Further, the crystal state has a greater light reflectivity than the amorphous state so that an information recorded on the phase-change optical disc can be reproduced.
In order to access information recorded on the phase-change optical disc, a pulse position modulation(PPM) system determines upon a position of a mark, and a pulse width modulation(PWM) system depending upon a width of mark. The PPM system dedicated to the PD has a disadvantage in that it deteriorates the recording density of information. On the other hand, the PWM system applicable to recording media, such as CD-R, DVD-RAM and so on, has an advantage in that it improves the recording density of information.
In the above modulation systems, the laser light beam having a shape of a write pulse WP and a write power as shown in FIG. 2A is irradiated onto the phase-change optical disc so as to record an information on the phase-change optical disc. In this case, since a recording layer in a region irradiated with a light beam is melted and then cooled, a new mark NMK in an amorphous state as shown in FIG. 2B is written into the phase-change disc. Further, a crystal particle ring CR in a crystal state always appears in the peripheral of the new mark NMK. This crystal particle ring CR is formed by heating the peripheral of the region irradiated with a light beam having the write power as shown in FIG. 2A into a temperature between the crystallization temperature and the melting point temperature and thus changing the same into a crystal state. For example, when a light beam having an aperture number of 0.5 and a wavelength of 825 nm is irradiated onto the phase-change optical disc, a crystal particle ring CR having a width of 0.3 .mu.m emerges around the mark. The crystal particle ring CR formed in this manner distinctly divides a boundary portion of the new mark NMK and the previous mark PMK such that an edge information of the new mark NMK is not influenced from a remaining signal due to the previous mark PMK.
The self-sharpening effect ("SSE") allowing the crystal particle ring to be formed as described above advantageously works on an information reproduced by the PPM system. Otherwise, the width of crystal particle ring CR is more enlarged as the size of mark becomes larger. In other words, as the size of mark is larger, the SSE becomes serious. This is caused by a fact that the phase-change optical disc has such a structure that a temperature of the recording layer 6 increases or decreases slowly and the top temperature thereof becomes relatively low. The crystal particle ring CR having a larger width in proportion to the size of mark weakens a characteristic of a signal reproduced from the phase-change optical disc using the PWM system. In view of this, there are required a new structure of phase-change optical disc that is suitable for weakening the SSE.